1. Field of the Invention
The present invention relates to a full color toner for use in developing electrostatic images formed by electrophotography, electrostatic recording, electrostatic printing, etc., a developer containing the full color toner and a carrier, a fixing method of fixing full color toner images on a recording medium such as paper, a container containing the full color toner, a process cartridge containing the full color toner or the developer, and an image forming apparatus to which the container is provided.
2. Discussion of the Background
Image forming methods for use in electrophotography typically include the following processes:    (1) Forming a latent electrostatic image on an image bearing member formed of, for example, a photoconductive material;    (2) Attaching charged toner particles to the latent electrostatic image to form a visualized toner image;    (3) Transferring the visualized toner image to a recording material such as paper; and    (4) Fixing the visualized toner image on the recording material before discharging the recording material outside;
Recently, the technology for use in photocopiers and printers using electrophotography has been rapidly extending from monochrome photocopying or printing to color photocopying or printing. Therefore the full color photocopier and printer market is expanding.
In color image forming based on full color electrophotography, all colors are typically reproduced by overlapping the layers of the three primary color toners, i.e., yellow, magenta and cyan toner, or of four color toners including the three primary color toners and black toner. To obtain a vivid and clear full color image having a good reproducibility, it is necessary to reduce light scattering by smoothing the surface of a fixed toner image to some degree. Because of this, typical full color photocopiers, etc. produce images having a gloss in the medium to high range, i.e., 10 to 50% in most cases.
As a method of fixing a dry toner image on a recording medium, a contact heating fixing method in which a roller or a belt having a smooth surface is heated to fix toner upon application of heat and pressure is normally adopted in many cases. This method is thermally efficient and fixes toner at a high speed, thereby providing gloss and transparency to color toner. Contrary to this advantage, offset phenomenon, in which part of a toner image attaches to the surface of a fixing roller and transfers to another image, occurs because the surface of the heated fixing roller is press-contacted with melted toner before detachment.
To prevent this offset phenomenon, a countermeasure has been adopted in which the surface of a fixing roller is formed of a material having a good releasability such as silicone rubber and fluorine containing resin and further a releasing oil such as silicone oil is applied to the surface of the fixing roller. Although this countermeasure is extremely effective to prevent toner offset, an additional device to supply the release oil is required, thereby increasing the size of the fixing device and cost. Therefore, this is not suitable in terms of reduction in size as a whole. Therefore, as for a monochrome toner, another method is-instead adopted in which no or a little amount of release oil is applied to a fixing roller (hereinafter referred to as an oilless method). In such an oilless method, viscosity and elasticity of the melted toner are increased by adjusting molecular weight distribution of a binder resin to prevent internal rupture of the melted toner and further a release agent such as wax is contained in the toner.
However, as mentioned above, in the case of a color toner, it is necessary to smooth the surface of an unfixed image to improve the color reproduction. Therefore, it is inevitable to reduce the viscosity and elasticity of a toner during melting. That is, relative to the case of a monochrome toner having a relatively low gloss, a color toner tends to offset so that it is difficult to adopt the oilless method mentioned above for a fixing device. In addition, when a release agent is contained in a toner, the attachment property of the toner is strengthened. As a result, the transferability of the toner to a transfer medium deteriorates. Further, this causes the problem that the release agent contained in the toner contaminates a friction charging member such as a carrier and reduces the chargeability of the friction charging member, resulting in deterioration of the durability of the friction charging member.
Typically binder resins such as polyester resins and epoxy resins, with which it is easy to obtain gloss with a low molecular weight, have been used in color toner. However, these resins contain a hydrophilic radical, which leads to a drawback in that the amount of charge therein greatly varies due to humidity. Further, there is a tendency that toner has recently been reduced in size to obtain quality images. However, polyester resins and epoxy resins have a drawback in that, polyester resins and epoxy resins are inferior to styrene resins, which have been used as a binder resin for use in monochrome toner, in terms of pulverization property.
Under these circumstances, unexamined published Japanese Patent Application No. (hereinafter referred to as JOP) H08-220808 describes a toner in which a combination of a linear type polyester resin having a softening point of from 90 to 120° C. and carnauba wax are used. JOP H09-106105 describes a toner formed of a resin and a wax which are compatible to each other and have a different softening point. JOP H09-304964 describes a toner in which the melting viscosity of each of a polyester resin and wax is regulated. JOP H10-293425 describes a toner containing a polyester resin having a softening point of from 90 to 120° C., rice wax, carnauba wax and silicone oil. JOP H05-61242 describes a polymerized toner containing wax therein. However, these toners fail to simultaneously and satisfactorily achieve anti-offset property, durability, stability in charging against moisture, and pulverization property when a fixing method in which no or even an extremely little amount of oil is applied is used.
Containing an ester wax as wax is also proposed to obtain a wide range of fixing temperature. JOP 2003-156876 describes a non-magnetic single-component developer including a polyester resin and an ester wax having an endothermic peak of 50 to 90° C. by DSC. Thereby, the developer has an excellent anti-offset property. But, since the ester wax and the polyester resin do not have good compatibility with each other, the ester wax and the polyester resin tend to have a phase separation so that the ester wax is released during developing, resulting in occurrence of carrier spent and filming. Therefore, durability of the developer is not good.
JOP 2000-181120 describes a developer including a polyester resin and an ester wax having an endothermic peak of 70 to 90° C. by DSC. Similarly, the developer does not improve the durability.
JOP 2002-23424 describes a color toner which contains an ester wax, a polyester resin, and a hybrid resin having a polyester and vinyl group based copolymer unit, and which has a molecular weight distribution parameter Mw/Mn by GPC not less than 100. Although the compatibility between the ester wax and polyester resin is improved by containing the hybrid resin, gloss property is not sufficient because Mw/Mn is not less than 100. Therefore, there is turbidity in color characteristics of the color toner and thus a good color saturation is not obtained.
Japanese Patent No. 3210174 describes a multi-functional polyester compound but a vinyl group based copolymer is used as a main binder resin so that its compatibility to the polyester resin is not improved.
JOP 2003-156880 describes a color toner containing a polyester resin and an olefin wax which has a molecular weight distribution parameter Mw/Mn not less than 50. Since the polyester resin has a good compatibility with the olefin wax, filming and spent can be improved by using the color toner. But the polyester resin is also compatible with the binder resin. Therefore, the release effect of the wax during fixing is insufficient. As a result, the molecular weight distribution parameter Mw/Mn is necessary to be not less than 50 to improve anti-hot-offset property. Consequently, sufficient gloss is not obtained again by using the color toner. In addition, there is turbidity in color characteristics of the color toner and thus a good color saturation is not obtained.
To solve these problems, there are many proposals in which hybrid resins are contained in a toner.
JOP 2002-23424 describes a mixture of a polyester or a styreneacryl resin and a hybrid resin. JOP 2001-272820 describes a mixture of two different hybrid resins. By these technologies, low temperature fixing property and anti-offset property can be improved. However, deterioration of gloss of color images and color reproducibility caused by containing a hybrid resin is not improved. These oilless fixing full color toner do not provide a combination of sufficient low temperature fixing property, good anti-hot-offset property, good gloss property, good color saturation and good durability without filming.
Further, especially a toner containing a wax tends to attach to each other and form agglomerated particles at a portion where stirring is performed in the developing portion. Especially in the case of a color toner, since color images are required to have a good gloss property, polyesters contained therein tend to have a low molecular weight. Thus, color toners further easily tend to form agglomerated particles in comparison with a toner dedicatedly for use in monochrome images. Further, since it is often that toner is actually stored in an environment in which the temperature is not less than 30° C. in actual photocopiers or printers, toner containing a wax more easily flocculates.
As for a two-component developer system, toner particles attach to each other and flocculate together by compressive contact between carriers while stirring the developer. In a one-component developer system, toner tends to flocculate together by pressure and friction heat when the toner is regulated to form a thin layer on a developing roller. Further, in both of a two-component developer system and a one-component system, heat generated by abrasion of axis of developer stirring blades and a screw causes the toner to be half melted and form agglomerated particles.
In the case of a two-component developer system, such toner agglomerated particles are developed or attached to an image portion, resulting in a large and thick dot thereon. When an image is transferred to a recording material, such agglomerated particles function as a spacer so that the corresponding portion in the image may white out.
In the case of a one-component developer system, such toner agglomerated particles sneak into a gap between a developing roller and a thin-layer forming blade, resulting in an abnormal image having a white streak.
Such an abnormal image tends to stand out in color in comparison with monochrome. In a situation in which quality images having a fine graduation property, a fine color reproducibility and a high definition are wanted, these abnormal images caused by agglomerated toner particles become a problem. Japanese Patents Nos. 3142297, 3073743, 3065073 and 3065032 describe a color toner formed of a linear polyester and a non-linear polyester. Thereby, fixing property and stability in charging of the color toner can be improved but improvement on agglomeration property thereof is not described.
JOP 2002-304006 describes a toner containing a polyester resin and a hybrid resin. The low temperature fixability, anti-hot-offset property and pulverization vulnerability are sufficiently improved by the toner. However, there is no description about gloss property, color reproducibility, and improvement on agglomeration property. A color toner having a sufficient low temperature fixing property and a sufficient anti-hot-offset property with improvement on gloss property, color reproducibility and agglomeration property has not been obtained.